1. Field of the Invention
The invention relates to excimer and molecular fluorine lasers, and particularly for generating line-narrowed DUV and VUV laser beams using lasers with lengthened electrodes for compensating one or more other factors tending to reduce the gain per resonator transit of the beam.
2. Discussion of the Related Art
Line-narrowed and/or line-selected excimer and molecular fluorine lasers are advantageously used in industrial applications such as optical microlithography for forming small electronic structures on silicon substrates, photoablation and micromachining, among others. Such lasers generally include a discharge chamber containing two or more gases such as a halogen and one or two rare gases. KrF (248 nm) and ArF (193 nm) excimer lasers are examples of excimer lasers that are typically line-narrowed and have gas mixtures, respectively, of krypton, fluorine and a buffer gas typically of neon, and argon, fluorine and a buffer gas of neon and or helium (see U.S. patent application Ser. Nos. 09/447,882, 09/734,459 and 09/513,025, 09/602,184, 09/629,256, 09/640,595, 60/162,735, 60/178,445, 09/715,803, 60/200,163, 09/584,420 and 60/215,933, which are assigned to the same assignee as the present application and are hereby incorporated by reference). The molecular fluorine (F2) laser has a gas mixture of fluorine and one or more buffer gases, and emits at least two lines around 157 nm, one of which may be selected, and narrowed, such that a very narrow linewidth VUV beam may be realized (see U.S. Pat. No. 6,157,152 and U.S. patent application Ser. Nos. 09/317,695, 09/130,277, 60/212,183, 09/482,698, 09/599,130, 60/173,993, 60/166,967, 60/657,396, 09/317,527, 60/170,919 and 60/212,301, which are assigned to the same assignee as the present application and are hereby incorporated by reference).
The KrF laser is perhaps the most commonly used laser for photolithographic applications today. The ArF laser and the F2 laser are, however, becoming and expected to become more prevalent for processing smaller structures due to their shorter wavelength emission spectra. Each of the ArF and F2 lasers exhibit lower gain and higher radiation losses, primarily due to optical absorption, than the KrF laser.
It is recognized in the present invention that it is desired to have an excimer or molecular fluorine laser system, particularly an ArF or F2 laser system, having enhanced gain characteristics. It is also recognized in the present invention that is also desired to have an excimer laser system exhibiting longer output emission pulses and/or longer inversion times. Longer pulses and/or longer inversion times may be achieved by using smaller pump intensities, or reduced electrical power deposition per discharge volume, and/or reduced halogen concentrations in the gas mixture. Such an excimer or molecular fluorine laser would feature a greater number of round trips for the beam in the resonator. Among the other advantages of such a laser would be enhanced line narrowing by the line-narrowing unit typically included in the resonator setups of these lasers. Reducing the volumetric power deposition and/or reducing the fluorine concentration, however, would also tend to cause the laser to exhibit reduced gain characteristics if not otherwise compensated.
It is an object of the invention to have an excimer or molecular fluorine laser having enhanced inherent gain characteristics.
It is another object of the invention to have an excimer or molecular fluorine laser system that exhibits longer output pulses and/or longer inversion times without sacrificing gain.
In accord with the above object, an excimer or molecular fluorine laser system is provided including a laser tube filled with a gas mixture at least including molecular fluorine and a buffer gas; a pulsed electrical discharge circuit; multiple electrodes within the laser tube defining a discharge area including a discharge width and connected with the pulsed discharge circuit for energizing the gas mixture, at least one of said electrodes comprising a base portion and a narrow center portion, said narrow center portion substantially carrying a periodic discharge current such that a discharge width is less than a width of the base portion, and the base portion is shaped to provide a selected electric field around the discharge area, and the discharge width is substantially the width of the narrow center portion, wherein as a result of said electrode configuration comprising said base portion and said narrow center portion, a discharge width is substantially 4 mm or less; one or more preionization units within the laser tube for ionizing the gas mixture prior to main discharges; a heat exchanger within the laser tube at least for cooling the gas mixture; a fan within the laser tube for circulating the gas mixture through the discharge area; a laser resonator including at least a portion of the discharge area within the laser tube for generating a pulsed laser beam having a desired energy and a bandwidth of substantially 0.5 pm or less.
An excimer or molecular fluorine laser is also provided including a discharge tube filled with a gas mixture. An electrical discharge circuit is connected to multiple electrodes housed within the discharge tube for energizing the gas mixture. A laser resonator including the discharge tube and preferably a line narrowing and/or selection unit for generating a line-narrowed and/or line-selected laser beam. At least one, and preferably both, of the main electrodes are significantly longer than conventional electrodes. For example, the main electrodes would be longer than 28 inches, and preferably 30 to 40 inches long or greater, an even as long or longer than 50 inches.
Advantageously, the lengthened main electrodes of the laser system provide enhanced gain. Particular application is for use with ArF and F2 laser systems, which as described above, tend to exhibit characteristically lower gains than other excimer laser systems such as the KrF laser.
Preferably, longer light pulses and/or longer inversion times are produced by reducing the electrical power deposition per discharge volume and/or by reducing the halogen concentration in the gas mixture. Also line-narrowing to 0.5 pm or less may be produced wherein loss of gain is compensated by the lengthened electrodes. The gas pressure within the laser tube may have a reduced pressure than conventional lasers, while the lengthened electrodes compensate the decrease in laser output energy associated with the lower pressure in the tube. The discharge width may be reduced, e.g., by reducing the width of the electrodes to enhance the clearing ratio of the gas mixture through the discharge for operation at higher repetition rates, or the output coupler may have a reduced reflectivity, each of which is compensated by the increased gain due to the lengthened electrodes. In each case, the reduced gain associated with this preferred feature is advantageously compensated by the lengthening of the electrodes.
Several references are cited herein which are, in addition to those references cited above and below herein, including that which is described as background, and the above invention summary, are hereby incorporated by reference into the detailed description of the preferred embodiments below, as disclosing alternative embodiments of elements or features of the preferred embodiments not otherwise set forth in detail below. A single one or a combination of two or more of these references may be consulted to obtain a variation of the preferred embodiments described in the detailed description below. Further patent, patent application and non-patent references are cited in the written description and are also incorporated by reference into the detailed description of the preferred embodiment with the same effect as just described.